Communications in an asynchronous cellular wireless network

ABSTRACT

Systems and techniques are disclosed for establishing a reference corresponding to the timing of a received signal from the first source, determining the timing for each received signal from a plurality of second sources, adjusting the reference to the timing of the received signal from one of the second sources, the timing of the received signal used to adjust the reference being closest in time to the unadjusted reference, and synchronizing a signal to the reference for transmission.

This application is a continuation of application Ser. No. 10/966,119,filed Oct. 14, 2004, now allowed, which is a continuation of applicationSer. No. 10/177,270, filed Jun. 21, 2002, which claims priority under 35U.S.C. §119(e) to provisional Application No. 60/337,472, filed Nov. 9,2001, the contents of which are incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to communications systems, and morespecifically, to systems and techniques for synchronizing acommunications device to an asynchronous network access point.

2. Background

Modern communications systems are designed to allow multiple users toshare a common communications medium. One such communications system isa code division multiple access (CDMA) system. A CDMA communicationssystem is a modulation and multiple access scheme based onspread-spectrum communications. In a CDMA communications system, a largenumber of signals share the same frequency spectrum and, as a result,provide an increase in user capacity. This is achieved by transmittingeach signal with a different code that modulates a carrier, and thereby,spreads the spectrum of the signal waveform. The transmitted signals areseparated in the receiver by a correlator that uses a corresponding codeto despread the signal's spectrum. The undesired signals, whose codes donot match, are not despread in bandwidth and contribute only to noise.

In a CDMA communications system, a user may access a network, orcommunicate with other users, through a network access point. A networkaccess point generally includes a radio network controller supportingmultiple nodes. For the purpose of this disclosure, the term “node” willbe used to refer to a node B, a base station, or any other similarcommunications station. Each node is assigned to serve all users withina region generally referred to as a cell or sector. Within any givenregion, a user may be in communication with any number of neighboringnodes as well as the node serving the region.

In some CDMA communications systems, the nodes are synchronized to oneanother. By way of example, the Naystar Global Positioning satellitenavigation system is often used to synchronize the nodes to a commontime reference. As a result, once a user acquires and synchronizes to anode, it can synchronously communicate with other nodes as it travelsfrom region to region. This is to be contrasted to an asynchronous CDMAcommunications system which may require the user to resynchronize todifferent nodes as it travels through various regions of coverage. Theresynchronization process should be performed quickly to minimizepotential interruptions in communications that may be perceived by theuser. Furthermore, it would be advantageous to minimize the time duringwhich a user is performing the resynchronization process because thisreduces the risk of dropped radio communication links to nodes otherthan the reference node, and it makes position estimates that are basedon propagation delay measurements more accurate.

SUMMARY

In one aspect of the present invention, a method of communicationsincludes establishing a reference corresponding to the timing of areceived signal from a first source, determining the timing for eachreceived signal from a plurality of second sources, adjusting thereference to the timing of the received signal from one of the secondsources, the timing of the received signal used to adjust the referencebeing closest in time to the unadjusted reference, and synchronizing asignal to the reference for transmission.

In another aspect of the present invention, an apparatus includes asearcher configured to establish a reference corresponding to the timingof a received signal from a first source, determine the timing for eachreceived signal from a plurality of second sources, adjust the referenceto the timing of the received signal from one of the second sources, thetiming of the received signal used to adjust the reference being closestin time to the unadjusted reference, and synchronize a signal to thereference for transmission.

In yet another aspect of the present invention, computer-readable mediaembodying a program of instructions executable by a computer performs amethod of communications, the method includes establishing a referencecorresponding to the timing of a received signal from a first source,determining the timing for each received signal from a plurality ofsecond sources, adjusting the reference to the timing of the receivedsignal from one of the second sources, the timing of the received signalused to adjust the reference being closest in time to the unadjustedreference, and synchronizing a signal to the reference for transmission.

In a further aspect of the present invention, an apparatus includesreference means for establishing a reference corresponding to the timingof a received signal from a first source, means for determining thetiming for each received signal from a plurality of second sources,adjustment means for adjusting the reference to the timing of thereceived signal from one of the second sources, the timing of thereceived signal used to adjust the reference being closest in time tothe unadjusted reference, and means for synchronizing a signal to thereference for transmission.

It is understood that other embodiments of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein it is shown and described only inexemplary embodiments of the invention by way of illustration. As willbe realized, the invention is capable of other and different embodimentsand its several details are capable of modification in various otherrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawings and detailed description are to beregarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention are illustrated by way of example, andnot by way of limitation, in the accompanying drawings, wherein:

FIG. 1 is a conceptual overview of an exemplary asynchronous CDMAcommunications system;

FIG. 2 is a diagram illustrating an exemplary downlink frame structurefor the a synchronization channel (SCH), a primary common controlphysical channel (P-CCPCH), and a common pilot channel (CPICH)transmitted by a node in an asynchronous CDMA communications system;

FIG. 3 is a functional block diagram illustrating the generation of thesynchronization channel (SCH), the primary common control physicalchannel (P-CCPCH), and the common pilot channel (CPICH) by an exemplarynode in an asynchronous CDMA communications system;

FIG. 4 is a conceptual overview of exemplary user equipment operating ina asynchronous CDMA communications;

FIG. 5 is a functional block diagram of exemplary user equipment for usein an asynchronous CDMA communications system; and

FIG. 6 is a functional block diagram of an exemplary searcher which canbe utilized in the user equipment of FIG. 5.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments in whichthe present invention can be practiced. The term “exemplary” usedthroughout this description means “serving as an example, instance, orillustration,” and should not necessarily be construed as preferred oradvantageous over other embodiments. The detailed description includesspecific details for the purpose of providing a thorough understandingof the present invention. However, it will be apparent to those skilledin the art that the present invention may be practiced without thesespecific details. In some instances, well known structures and devicesare shown in block diagram form in order to avoid obscuring the conceptsof the present invention.

Although various aspects of the present invention will be described inthe context of a CDMA communications system, those skilled in the artwill appreciate that these aspects are likewise suitable for use invarious other communications environments. Accordingly, any reference toa CDMA communications system is intended only to illustrate theinventive aspects of the present invention, with the understanding thatsuch inventive aspects have a wide range of applications.

FIG. 1 is a functional block diagram of an asynchronous CDMAcommunications system. A radio network controller 102 can be used toprovide an interface between a network 104 and all nodes 106 a-ddispersed throughout a geographic area. The geographic area is generallydivided into regions known as cells or sectors. A node is generallyassigned to serve all users in a region. User equipment 108 may accessthe network 104, or communicate with other user equipment, through oneor more nodes under control of the radio network controller 102. In thecase where the user equipment 108 is in communication with more than onenode, the user equipment 108 selects a reference node as asynchronization source for its transmissions to all nodes. As the userequipment 108 moves away from the reference node, the user equipment mayultimately need to select a new node as a synchronization source. Theuser equipment can select the node that requires the least amount ofslewing to resynchronize its transmissions. Slewing refers to theprocess of adjusting the transmit signal timing of the user equipment inorder to synchronize it with a new synchronization source. To minimizeslewing, the user equipment can select the node whose signals arereceived closest in time to the previous received signals from thereference node.

An exemplary asynchronous CDMA communications system may be designed tosupport the FDD mode of operation of the standard offered by aconsortium named “3rd Generation Partnership Project” referred to hereinas 3GPP, and embodied in a set of documents including Document Nos. 3GPPTS21.101 3GPP TS 25.211, 3GPP TS 25.212, 3GPP TS 25.213, 3GPP TS 25.214,and 3G TS 25.133, referred to herein as the W-CDMA standard. The W-CDMAstandard is expressly incorporated herein by reference. W-CDMAspecifications issued by 3GPP are public record and are well known inthe art. W-CDMA (also known as UTRA-FDD) is adopted and issues as aregional standard by various standardization bodies as for instance bythe European Telecommunications Standard Institute (ETSI). The 3GPPspecifications describe the use of a combination of the physicalchannels SCH and CPICH transmitted by each node in a W-CDMAcommunications system. SCH and CPICH can be used by the user equipmentto synchronize to different nodes as the user equipment moves throughoutthe coverage area.

FIG. 2 is a diagram illustrating a downlink frame structure for thephysical channels SCH, CPICH, and P-CCPCH transmitted by a node in aW-CDMA communications system. A frame 202 can be any duration dependingon the particular application and overall design constraints. In thedescribed exemplary W-CDMA communications system, the frame duration isten milliseconds and includes 38,400 chips. The frame 202 can be dividedinto fifteen slots 204 with each slot having 2560 chips. Each slot 204can be further partitioned into ten parts 206 with each part having 256chips.

The SCH and P-CCPCH are time-multiplexed. The SCH is only transmittedduring the first part of each slot 204 and the P-CCPCH is transmittedonly during parts 2 through 10 of each slot. The CPICH is transmitted inparallel to the SCH and the P-CCPCH. The frame timing for the SCH,P-CCPCH and CPICH is identical. The SCH is sub-divided into a primarySCH carrying a primary synchronization code (PSC) sequence, and asecondary SCH carrying a secondary synchronization code (SSC) sequence.The PSC and the SSC sequences are orthogonal to each other. They aregenerated using generalized hierarchical Golay sequences and Hadarmardsequences and are transmitted on top of one another. The PSC sequence isthe same sequence for every slot and for every node in the coveragearea. The SSC sequence can be one of fifteen possible sequences in eachslot. The P-CCPCH carries broadcast data such as the identity of thetransmitting node and other information that may be of general use toall user equipment in communication with that node. In parallel to theSCH and P-CCPCH, the CPICH is continuously transmitted. The CPICHcarries an a priori known pilot signal. The pilot signal can be used bythe user equipment to synchronize to the node and serves as a phasereference in order to coherently demodulate data transmitted to the userequipment once the user equipment is synchronized to the node and hassuccessfully completed an access attempt to the system.

The pilot signal contains no data and is often characterized as anunmodulated spread spectrum signal. The pilot signal from each node isgenerally spread with the same orthogonal code but scrambled with adifferent node-specific primary scrambling code. The primary scramblingcode is truncated at the end of each CPICH frame, and then repeats fromthe beginning at the start of each frame. In the exemplary W-CDMAcommunications system there are 512 possible primary scrambling codesfor a given node. Also the P-CCPCH is scrambled with the primaryscrambling code. The primary scrambling code used by a node is not apriori known by the user equipment.

FIG. 3 is a functional block diagram illustrating the generation of theSCH, P-CCPCH, and the CPICH channel by the node. A PSC generator 302 canbe used to generate the PSC sequence comprising a predetermined 256 chipsequence that is used in the user equipment for slot timing acquisitionof the node. A SSC generator 304 can be used to generate the SSCsequence. The SSC sequence serves two functions. First, the SSC sequenceis used in the user equipment to identify the frame timing of the node.Second, the SSC sequence also provides a code group identifier whichidentifies a group of eight possible primary scrambling codes. In W-CDMAcommunications systems using 512 possible primary scrambling codes,there are sixty-four code group identifiers. The SSC generator 304 firstmaps the group identifier into one of sixty-four possible fifteenelement code words, and then maps each code word element which can havesixteen different possible values into one of sixteen possible 256 chipsequences. Each of the sixteen possible 256 chip sequences, as well asthe PSC sequence, are orthogonal to one another. A summer 306 can beused to combine each of the fifteen 256 chip SSC sequences with the PSCsequence. A puncture element 308 can be used to puncture the PSC and SSCsequences from the summer 306 into the first part of the each slot. Aprimary scrambling code generator 310 can be used to generate theprimary scrambling code for the node. The broadcast data can then bescrambled with the primary scrambling code using a multiplier 312 andpunctured into parts 2-10 of each slot with the puncture element 308. Anorthogonal code generator 314 can be used to generate the CPICH. TheCPICH can then be scrambled with the primary scrambling code using amultiplier 316. The scrambled CPICH can be combined with the SCH and thescrambled P-CCPCH from the puncture element 308 as well as otherdownlink channels using a summer 318.

FIG. 4 is a functional block diagram of exemplary user equipmentoperating in an W-CDMA communications environment. An RF analog frontend (AFE) 402 coupled to one or more antennas 404 can be used to supporta radio communication link a node. In the receive mode, a signaltransmitted from a node is coupled from the antenna 404 to the AFE 402.The AFE filters and amplifies the signal, downconverts the signal tobaseband, and digitizes the baseband signal.

The digital baseband signal can be provided to a searcher 406 for thepurposes of acquisition and synchronization. The acquisition of thenode's slot timing involves a search through the digital baseband signalto find the PSC sequence embedded in the SCH. This can be achieved bycorrelating the digital baseband signal with a locally generated PSCsequence. In a manner to be described in greater detail later, the frametiming can be extracted from the SSC sequences, and used to determinewhich code group out of the sixty-four possible code groups does theprimary scrambling code of the node belong to. Knowing the code group ofthe primary scrambling code, the searcher 406 can determine whichprimary scrambling code is actually used by the node. This could beachieved by correlating the digital baseband signal with eight versionsof the a priori known pilot signal that are generated by scrambling withthe eight possible primary scrambling codes of the code group andselecting the one which results in the highest energy at the correlationoutput. With the information about the slot timing, the frame timing andthe primary scrambling code, the user equipment is able to use the CPICHfor channel estimation and coherent demodulation of data transmitted tothe user equipment.

The digital baseband signal can also be provided to a receiver 407. Thereceiver includes a demodulator 408 and a decoder 410. The demodulator408 can be implemented in a variety of fashions. By way of example, inW-CDMA communications systems, or any other type of communicationssystem which uses diversity techniques to combat fading, a rake receivermay be used. The rake receiver typically utilizes independent fading ofresolvable multipaths to achieve diversity gain. This can be achievedthrough a combined effort between the searcher 406 and the rakereceiver. More specifically, the searcher 406 can be configured toidentify strong multipath arrivals of the pilot signal. Fingers can thenbe assigned by the searcher 406 to identify the timing offsets of themultipaths. The fingers can be used by the rake receiver as a timingreference to correlate the traffic for each anticipated multipathreflection. The separate correlations can then be coherently combinedand provided to the decoder 410 for deinterleaving, decoding, and framecheck functions.

The user equipment may also include a transmitter 411 to support thetransmit mode. The transmitter 411 includes a data queue 412, an encoder414, and a modulator 416. The data queue 412 can be used to buffer datathat the user equipment intends to send to the node. The frame timinginformation derived by the searcher 406 can be used to release trafficfrom the data queue 412 with a time offset from the correspondingdownlink frame. In W-CDMA communications systems, data is released fromthe data queue 412 in a manner that creates a 1024 chip offset from thereception of a frame via the first detectable multipath of the downlinkfrom the reference node relative to the transmission of thecorresponding frame in the uplink. However, any offset may be useddepending on the particular application and the overall designparameters.

The data from the data queue 412 can be provided to an encoder 414 forencoding, interleaving and frame check functions. The encoded data fromthe encoder 414 can then be provided to a modulator 416 which spreadsthe data with orthogonal codes. The modulated data can then be providedto the AFE 402 where it is filtered, upconverted, amplified and coupledto the antenna 404.

FIG. 5 is a diagram of a user equipment operating in a multiple nodeW-CDMA communications environment. In the W-CDMA communicationsenvironment, four nodes 502 a-d are shown. Each node 502 a-d transmits apilot signal throughout its respective coverage region 504 a-d. Thepilot signal transmitted by each node 502 a-d is spread with the sameorthogonal code but scrambled with different primary scrambling codes.The primary scrambling code allows the pilot signals to be distinguishedfrom one another thus distinguishing the originating nodes 502 a-d. Userequipment 506 is shown moving through different coverage regions by aseries of broken lines. The user equipment 506 is shown initially movingthrough the first coverage region 504 a. It is assumed that the userequipment 506 is attempting to establish communications with thenetwork. From the first coverage region 504 a, the user equipment 506searches for the SCH and the CPICH for acquisition and synchronizationas described earlier. After this is accomplished, the user equipment 506determines the quality of the radio communication links to the nodes bymeasuring the strength of the pilot signals from the nodes. When thestrength of the pilot signal exceeds a threshold, in this case the pilotsignal from the first node 502 a, the user equipment 506 attempts toaccess that node 502 a. Depending on the resources available, the node502 a may establish a radio communication link to the user equipment 506for downlink traffic transmissions. After the access attempt iscompleted successfully, the user equipment 506 adds that node 502 a toits active set and establishes a radio communication link to transmittraffic to that node. At this point only the node 502 a is a member ofthe active set of the user equipment 506. The user equipment 506 alsouses that node 502 a as a reference to synchronize its uplink frames.

As the user equipment 506 moves into an area where the first and secondcoverage regions 504 a-b overlap, the strength of the pilot signal fromthe second node 502 b increases until it exceeds the threshold. As aresult, the second node 502 b may be added to the active set of the userequipment 506 and another radio communication link established. In thatcase, the user equipment 506 communicates with both the first and secondnodes 502 a-b, but the transmission of the uplink frames by the userequipment 506 remains synchronized to the reference node 502 a.

As the user equipment 506 moves out of the first coverage region 504 a,the strength of the pilot signal from the first node 502 a decreasesuntil it drops below the threshold causing the reference node 502 a tobe removed from the active set of the user equipment 506. In at leastone embodiment, the reference node is not removed from the active setimmediately upon the pilot signal strength dropping below the threshold.Rather, the pilot signal strength should remain below the threshold fora predetermined time before the reference node is removed from theactive set. This approach decreases the likelihood that the referencenode is removed from the active set of the user equipment because ofspurious signal level fluctuations. Once the reference node 502 a isremoved from the active set of the user equipment 506, the radiocommunication link between the two is torn down, and the user equipment506 resynchronizes the timing of its uplink frames to the downlinkframes from the second node 502 b using the frame timing informationextracted from the PSC and SSC sequences embedded in the SCH channel andthe multipath timing estimated from the CPICH of the second node 502 b.The second node 502 b has now become the reference node.

As the user equipment 506 moves further towards its final destination,it moves into an area where the second, third and fourth coverageregions 504 b-d overlap. In this area, the strength of the pilot signalsfrom the third and fourth nodes 502 c-d increase until each of themexceeds the threshold. As a result, the third and fourth nodes 502 c-dmay be added to the active set of the user equipment 506, and a radiocommunication link established between the user equipment 506 and eachof the third and fourth nodes 502 c-d. In that case, the user equipment506 communicates with the second, third and fourth nodes 502 b-d, butthe transmission of the uplink frames remains synchronized to the secondnode 502 b.

With the user equipment 506 synchronized to the second node 502 b, anambiguity may arise as to which of the two nodes 502 c-d the userequipment 506 should select as the reference node as it moves out of thesecond coverage region 504 b. This ambiguity can be resolved in anynumber of ways. By way of example, the user equipment 506 canresynchronize to the node that requires the least amount of slewing.More specifically, the user equipment 506 can resynchronize to the nodefor which the first multipath arrival of the start of a downlink frameis closest in time to the start of the first multipath arrival of thesame downlink frame previously received from the second node 502 b.

FIG. 6 is a functional block diagram of an exemplary searcher which canbe used in the user equipment of FIG. 4. The searcher includes a PSCdetector 602. Since the PSC sequence is the same for each slot, the PSCdetector 602 can estimate the slot timing by correlating the receiveddigital baseband signal with a locally generated replica of the PSCsequence by means well known in the art.

An SSC detector 604 can be used to decode the SSC sequence by means wellknown in the art. More specifically, the SSC detector 604 correlates theSSC sequence in each slot (which can be one of sixteen possiblesequences) over one or more frames to determine the sixteen code wordelements. Based on the resultant code word, the SSC detector 604 candetermine the first slot in the frame, and with the slot timinginformation from the PSC detector 602, can determine the frame timing.The SSC detector 604 can also de-map the code word into the code groupidentifier for the node's primary scrambling code.

A pilot detector 606 can be used to correlate the digital basebandsignal with a locally generated scrambled orthogonal code. An orthogonalcode generator 608 can be used to generate the eight possible scrambledorthogonal codes for the code group to which the node is assigned basedon the code group identifier from the SSC detector 604. By means wellknown in the art, one or more slots of the received digital basebandsignal can be correlated with each of the eight possible orthogonalcodes until the pilot signal is detected.

As a result of this process, multiple copies of the pilot signal fromthe node may be detected at different times due to multipathreflections. A timing generator 610 can be used to detect the multipathsof the pilot signal and assign fingers to the rake receiver (not shown)accordingly. In communications involving multiple nodes, the frametiming for each node in the active set can be provided to a selector612. The timing generator 610 can be used to select the frame timing forthe first multipath arrival from the reference node. The selected frametiming is provided to an offset generator 614 to delay the uplinktransmission from the reception of the corresponding downlink frame. Inthe described exemplary embodiment, the delay is 1024 chips, althoughany delay could be used depending on the particular application and theoverall design constraints. The offset generator 614 can be used todelay the release of the data from the data queue 412 (see FIG. 4)accordingly. The offset generator 614 should be set for something lessthan the desired delay to account for the processing delay of thesearcher 406, encoder 414, modulator 416, and AFE 402 (see FIG. 4).

During the traffic communications between the node and the userequipment, the pilot detector 606 continues to search for new pilotsignals. Once a new pilot signal with sufficient strength is detected,the originating node can be added to the active set. At the same time,the pilot detector 606 continues to monitor the pilot signals from theactive nodes using the specific primary scrambling codes established foreach pilot signal during acquisition. Should the pilot signal from anynode fall below a predetermined threshold for an extended period oftime, then that node should be removed from the active set. In the eventthat the pilot signal from the reference node falls below thepredetermined threshold, the pilot detector 606 can cause the timinggenerator 610 to select a new node as a timing reference from the activeset. Assuming two or more nodes remain active, the timing generator 610should select the node that requires the least amount of slewing. Thismeans that the timing generator 610 should select the node in which thefirst multipath arrival for the downlink frame is closest in time to thefirst multipath arrival for the downlink frame previously transmittedfrom the former reference node. This selection criteria should beemployed even if the frame timing information from the SCH for theformer reference node is no longer available. This can be accomplishedin a variety of ways. In user equipment having a rake receiver, thefinger assignments for the former reference node can be used to selectthe appropriate node for resynchronization. Alternatively, the frametiming for the uplink transmission can be used to select the appropriatenode for resynchronization. Specifically, with regard to the latterapproach, the timing generator 610 can be used to establish a referencecorresponding to the frame timing of the uplink transmission. Thereference can be moved earlier in time by 1024 chips to derive the firstmultipath arrival for the downlink frame from the former reference node.This approach is an attractive solution because the frame timing for theuplink transmission can be determined even if the finger assignments forthe former reference node are no longer present.

Regardless of the methodology, the timing generator 610 will select anew node as a reference to resynchronize its transmissions. The selector612 can be used to select the frame timing for the first multipatharrival from the reference node. The selected frame timing can beprovided to an offset generator 614 to delay the uplink transmissionfrom the reception of the corresponding downlink frame from the newreference node. A trigger from the offset generator 614 can be used torelease traffic from the data queue 412 in the transmitter 411 (see FIG.4). The adjustment of the uplink transmission timing duringresynchronization should be done smoothly rather than instantaneously.Otherwise the node receivers might not be able to track this change oftransmit timing.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions herein. A generalpurpose processor may be a microprocessor, but in the alternative, theprocessor may be any conventional processor, controller,microcontroller, or state machine. A processor may also be implementedas a combination of computing devices, e.g. a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The methods or algorithms described in connection with the embodimentsdisclosed herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of selecting a synchronizing source from a first region to asecond region in a wireless communication system, comprising: selectinga first reference node from said first region as said synchronizingsource; entering into said second region having a plurality of nodes,wherein each of said plurality of nodes from said second region sends asignal having at least one downlink frame; and selecting a secondreference node from said plurality of nodes from said second region assaid synchronizing source such that the first multipath arrival of thestart of said at least one downlink frame from said second node isclosest in time to the start of the first multipath arrival of thecorresponding downlink frame from said first reference node.
 2. Themethod of claim 1 further including transmitting an uplink signal tosaid second reference node within a predetermined time after selectingsaid second reference node as said synchronizing source.
 3. The methodof claim 1 wherein said downlink frames are frames from respective pilotchannels of said first and second reference nodes.
 4. The method ofclaim 1 wherein said downlink frames are frames from respective physicalchannels of said first and second reference nodes.
 5. The method ofclaim 1 wherein said downlink frames are asynchronous with respect toeach other.
 6. An apparatus for of selecting a synchronizing source froma first region to a second region in a wireless communication system,comprising: means for selecting a first reference node from said firstregion as said synchronizing source; means for entering into said secondregion having a plurality of nodes, wherein each of said plurality ofnodes from said second region sends a signal having at least onedownlink frame; and means for selecting a second reference node fromsaid plurality of nodes from said second region as said synchronizingsource such that the first multipath arrival of the start of said atleast one downlink frame from said second node is closest in time to thestart of the first multipath arrival of the corresponding downlink framefrom said first reference node.
 7. The apparatus of claim 6 furthercomprising means for transmitting an uplink signal to said secondreference node within a predetermined time after selecting said secondreference node as said synchronizing source.
 8. The apparatus of claim 6wherein said downlink frames are frames from respective pilot channelsof said first and second reference nodes.
 9. The apparatus of claim 6wherein said downlink frames are frames from respective physicalchannels of said first and second reference nodes.
 10. The apparatus ofclaim 6 wherein said downlink frames are asynchronous with respect toeach other.
 11. A mobile station for selecting a synchronizing sourcefrom a first region to a second region in a wireless communicationsystem, comprising: circuitry configured to: select a first referencenode from said first region as said synchronizing source; enter intosaid second region having a plurality of nodes, wherein each of saidplurality of nodes from said second region sends a signal having atleast one downlink frame; and select a second reference node from saidplurality of nodes from said second region as said synchronizing sourcesuch that the first multipath arrival of the start of said at least onedownlink frame from said second node is closest in time to the start ofthe first multipath arrival of the corresponding downlink frame fromsaid first reference node.
 12. The mobile station of claim 11 furthercomprising circuitry to transmit an uplink signal to said secondreference node within a predetermined time after selecting said secondreference node as said synchronizing source.
 13. The mobile station ofclaim 11 wherein said downlink frames are frames from respective pilotchannels of said first and second reference nodes.
 14. The mobilestation of claim 11 wherein said downlink frames are frames fromrespective physical channels of said first and second reference nodes.15. The mobile station of claim 11 wherein said downlink frames areasynchronous with respect to each other.
 16. A computer program product,comprising: a computer-readable medium physically embodied withcomputer-readable program code for: selecting a first reference nodefrom said first region as said synchronizing source; entering into saidsecond region having a plurality of nodes, wherein each of saidplurality of nodes from said second region sends a signal having atleast one downlink frame; and selecting a second reference node fromsaid plurality of nodes from said second region as said synchronizingsource such that the first multipath arrival of the start of said atleast one downlink frame from said second node is closest in time to thestart of the first multipath arrival of the corresponding downlink framefrom said first reference node.
 17. The computer program product ofclaim 16 further including computer-readable program code fortransmitting an uplink signal to said second reference node within apredetermined time after selecting said second reference node as saidsynchronizing source.
 18. The computer program product of claim 16wherein said downlink frames are frames from respective pilot channelsof said first and second reference nodes.
 19. The computer programproduct of claim 16 wherein said downlink frames are frames fromrespective physical channels of said first and second reference nodes.20. The computer program product of claim 16 wherein said downlinkframes are asynchronous with respect to each other.